Type-safe builder

BuilderExample.scala

/**
 * Type-safe builder pattern.
 *  - setters can be called in any order
 *  - setting something twice is a compile error
 *  - calling build on an incomplete object is compile error 
 *  - custom error messages are emitted to describe the exact problem
 */
object BuilderExample {
  def main(args: Array[String]): Unit = {
    val p =
      Person.builder
        .age(26)
        .address("London")
        .name("Joe")
        .build
  }
}

case class Person(name: String, age: Int, address: String)

sealed trait Field[+T]
case class Set[+T](value: T) extends Field[T]
sealed trait Unset extends Field[Nothing]
case object Missing extends Unset

object Person {
  @implicitNotFound(msg = "You're trying to set the 'name' field twice")
  type is0[-From, +To] = <:<[From, To]
  @implicitNotFound(msg = "You're trying to set the 'age' field twice")
  type is1[-From, +To] = <:<[From, To]
  @implicitNotFound(msg = "You're trying to set the 'address' field twice")
  type is2[-From, +To] = <:<[From, To]
  @implicitNotFound(msg = "Object is incomplete")
  type is3[-From, +To] = <:<[From, To]

  case class Builder[NameT <: Field[String], AgeT <: Field[Int], AddressT <: Field[String]](
    _name: NameT, _age: AgeT, _address: AddressT) {

    def name(n: String)(implicit ev: NameT is0 Unset) =
      this.copy(_name = Set(n))

    def age(a: Int)(implicit ev: AgeT is1 Unset) =
      this.copy(_age = Set(a))

    def address(a: String)(implicit ev: AddressT is2 Unset) =
      this.copy(_address = Set(a))

    def build(implicit ev1: NameT is3 Set[String], ev2: AgeT is3 Set[Int], ev3: AddressT is3 Set[String]) =
      Person(_name.value, _age.value, _address.value)

  }

  val builder = Builder(Missing, Missing, Missing)

}